The present invention relates to a process for recovering elemental phosphorus from pond sludge which comprises heating a mixture of the sludge and water, washing the sludge at elevated temperatures, and reheating the sludge. Elemental phosphorus can be readily recovered from a concentrated phosphorus layer which forms during the treatment process.
Elemental phosphorus is produced commercially in an electric arc furnace by the thermal reduction of phosphate ore in the presence of coke and silica. The gaseous mixture produced by this process comprises phosphorus, water vapor, fluorosilicates, and other gaseous impurities, as well as entrained fine solid particles. This gas is condensed by contact with a water spray to form an aqueous phosphoruscontaining suspension. Upon settling, the suspension is allowed to stratify into three layers: a bottom layer of high purity liquid elemental phosphorus (i.e. 99% by weight or more), a top layer of water, and an intermediate layer of phosphorus sludge.
Phosphorus sludge is a semi-solid gel-like material having a relatively high viscosity when at rest. When in motion, however, the material acts as a liquid and the viscosity can drop to relatively low levels. The phosphorus sludge is considered to be an emulsion which is believed to be formed by the bonding of fine dirt particles to phosphorus particles. The amount of sludge formed in any particular process appears to be related to the quantity of solid particles in the gas stream fed to the condenser. This is in turn affected by the physical condition (particle size and fines content) of the burden constituents, by the operating conditions of the furnace, and by the effectiveness of any dust collectors in the furnace system. A detailed description of the composition and properties of phosphorus sludge is contained in U.S. Pat. No. 4,492,627, to Crea, issued Jan. 8, 1985.
The various waste water streams which are generated during phosphorus production are typically discharged in holding ponds located near the plant. This sludge, which is hereinafter referred to as "pond sludge", is an emulsion of phosphorus, solids and water having a relatively low phosphorus content and varying amounts of water and dirt, with the major portion of the pond sludge comprising water. A typical pond sludge composition contains about 20% phosphorus, 20% dirt, and 60% water, all by weight.
Although the total amount of phosphorus contained in a typical pond is substantial, reclamation of the phosphorus is a difficult proposition, both from the standpoint of physical and/or chemical treatment, as well as product economics. The emulsion is difficult to break using mechanical means since the phosphorus and dirt are chemically bound and difficult to separate. Chemical treatment can create, environmental problems and is therefore generally avoided. In addition, the relatively high concentration of water makes evaporation of the pond sludge economically undesirable.
Previous attempts to reclaim phosphorus from pond sludge have used mechanical devices such as centrifugal separators. See, for instance, U.S. Pat. No. 4,492,627, which describes the use of a plurality of centrifugal separators operated in tandem to separate phosphorus from the dirt contained in the sludge. The use of centrifugal separators, however, is associated with certain disadvantages. The product must still be filtered which generally necessitates an intermediate settling step requiring several hours or days. The centrifuge waste also contains large amounts of residual phosphorus, and is therefore relatively inefficient.
U.S. Pat. No. 4,399,037, issued 1983 to Diskowski et al, describes a process for removing yellow phosphorus from phosphorus-containing waste water using a combination of mechanical, chemical and thermal treatment methods. The waste water is filtered in a centrifugal filter at a temperature of 50.degree. C. to 70.degree. C. and a pH in the range of 2.0 to 3.5, the filtrate is allowed to separate into a phosphorus layer and a treated water layer, and the waste water is finally treated with an oxidant and neutralized by reaction with calcium metasilicate.
As will be readily appreciated from the foregoing, an improved process for recovering elemental phosphorus from phosphorus sludge is highly desirable. Such a process should have the attributes of both technical simplicity and economic feasibility. The economics of any such process depend to a large extent on the efficient removal of water from the sludge prior to evaporation. Direct evaporation of the sludge is not cost effective since a large proportion of the energy spent during evaporation is used to remove water. In addition, the evaporators require extensive routine maintenance and repair, and the waste from the evaporator still contains large quantities of elemental phosphorus. Furthermore, the evaporators have capacity limitations and are not designed for handling charges of material containing concentrations of phosphorus below about 25%.
It is therefore a principal object of this invention to provide an efficient and relatively inexpensive process for recovering elemental phosphorus from phosphorus-containing sludge.